Method for Preparing White Paper

ABSTRACT

Object of the invention is the method for preparing white paper which comprises adding to the pulp mass an aqueous formulation comprising
     a) at least one optical brightener of formula (I)   

     
       
         
         
             
             
         
       
         
         
           
             in which 
             the anionic charge on the brightener is balanced by a cationic charge composed of one or more identical or different cations selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4  linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4  linear or branched hydroxyalkyl radical, ammonium which is, di-, tri- or tetrasubstituted by a mixture of C 1 -C 4  linear or branched alkylradical and linear or branched hydroxyalkyl radical or mixtures of said compounds, wherein 
             R 1  and R 1 ′ may be the same or different, and each is hydrogen, C 1 -C 4  linear or branched alkyl, C 2 -C 4  linear or branched hydroxyalkyl, CH 2 CO 2   − , CH 2 CH 2 CONH 2  or CH 2 CH 2 CN, 
             R 2  and R 2 ′ may be the same or different, and each is C 1 -C 4  linear or branched alkyl, C 2 -C 4  linear or branched hydroxyalkyl, CH 2 CO 2   − , CH(CO 2   − )CH 2 CO 2   − , CH(CO 2   − )CH 2 CH 2 CO 2   − , CH 2 CH 2 SO 3   − , CH 2 CH 2 CO 2   − , CH 2 CH(CH 3 )CO 2   − , benzyl, or 
             R 1  and R 2  and/or R 1 ′ and R 2 ′, together with the neighboring nitrogen atom signify a morpholine ring; 
             R 3  signifies hydrogen, —CO 2   −  or —SO 3   −  and 
             p is 0, 1 or 2, 
             and 
           
         
         b) at least one shading dye of formula (II) 
       
    
     
       
         
         
             
             
         
       
         
         
           
             in which 
             R 4  signifies H, methyl or ethyl, 
             R 5  signifies paramethoxyphenyl, methyl or ethyl, 
             M signifies a cation selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4  linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4  linear or branched hydroxyalkyl radical, ammonium which is, di-, tri- or tetrasubstituted by a mixture of C 1 -C 4  linear or branched alkylradical and linear or branched hydroxyalkyl radical or mixtures of said compounds, 
           
         
         c) optionally one or more auxiliaries and 
         d) water.

The instant invention relates to a method for preparing white paper which comprises adding to the pulp mass an aqueous formulation consisting essentially of derivatives of diaminostilbene optical brighteners, shading dyes, optionally auxiliaries, and water.

BACKGROUND OF THE INVENTION

It is well known that the whiteness and thereby the attractiveness of papers can be improved by the addition of optical brighteners and shading dyes to the pulp mass prior to sheet formation.

However, the decrease of the brightness while using shading dyes is a widely known problem.

WO 0218705 A1 however teaches that the use of shading dyes, while having a positive effect on whiteness, has a negative impact on brightness. The solution to this problem is to add additional optical brightener, the advantage claimed in WO 0218705 A1 being characterized by the use of a mixture comprising at least one direct dye (exemplified by C.I. Direct Violet 35) and at least one optical brightener.

Surprisingly, we have now discovered certain shading dyes which have a strongly positive effect on whiteness while having little or no effect on brightness, and which can be used in aqueous formulations comprising optical brighteners, optionally auxiliaries, and water in order to enable the papermaker to reach high levels of whiteness and brightness.

Therefore, the goal of the present invention is to provide a method for preparing white paper which comprises adding to the pulp mass an aqueous formulation containing derivatives of diaminostilbene optical brighteners, certain shading dyes, optionally auxiliaries, and water. The paper prepared according to the present invention affords enhanced high whiteness levels while avoiding the disadvantages characterized by the use of shading dyes (loss of brightness) or pigments (lower whiteness build) recognized as being state-of-the-art.

DESCRIPTION OF THE INVENTION

The present invention therefore provides a method for preparing white paper which comprises adding to the pulp mass an aqueous formulation comprising

-   -   (a) at least one optical brightener of formula (I)

-   -   -   in which         -   the anionic charge on the brightener is balanced by a             cationic charge composed of one or more identical or             different cations selected from the group consisting of             hydrogen, an alkali metal cation, alkaline earth metal,             ammonium, ammonium which is mono-, di-, tri- or             tetrasubstituted by a C₁-C₄ linear or branched alkyl             radical, ammonium which is mono-, di-, tri- or             tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl             radical, ammonium which is, di-, tri- or tetrasubstituted by             a mixture of C₁-C₄ linear or branched alkylradical and             linear or branched hydroxyalkyl radical or mixtures of said             compounds,         -   R₁ and R₁′ may be the same or different, and each is             hydrogen, C₁-C₄ linear or branched alkyl, C₂-C₄ linear or             branched hydroxyalkyl, CH₂CO₂ ⁻, CH₂CH₂CONH₂ or CH₂CH₂CN,         -   R₂ and R₂′ may be the same or different, and each is C₁-C₄             linear or branched alkyl, C₂-C₄ linear or branched             hydroxyalkyl, CH₂CO₂ ⁻, CH(CO₂)CH₂CO₂, CH(CO₂)CH₂CH₂CO₂ ⁻,             CH₂CH₂SO₃ ⁻, CH₂CH₂CO₂ ⁻, CH₂CH(CH₃)CO₂ ⁻, benzyl, or         -   R₁ and R₂ and/or R₁′ and R₂′, together with the neighboring             nitrogen atom signify a morpholine ring         -   R₃ signify hydrogen, —CO₂ or —SO₃ ⁻ and         -   p is 0, 1 or 2,

    -   (b) at least one shading dye of formula (II)

-   -   -   in which         -   R₄ signifies H, methyl or ethyl,         -   R₅ signifies paramethoxyphenyl, methyl or ethyl,         -   M signifies a cation selected from the group consisting of             hydrogen, an alkali metal cation, alkaline earth metal,             ammonium, ammonium which is mono-, di-, tri- or             tetrasubstituted by a C₁-C₄ linear or branched alkyl             radical, ammonium which is mono-, di-, tri- or             tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl             radical, ammonium which is, di-, tri- or tetrasubstituted by             a mixture of C₁-C₄ linear or branched alkylradical and             linear or branched hydroxyalkyl radical or mixtures of said             compounds,

    -   (c) optionally one or more auxiliaries and

    -   (d) water.

In compounds of formula (I) in which R₃ is —SO₃ ⁻ and p is 1, the —SO₃ ⁻ group is preferably in the 4-position of the phenyl ring.

In compounds of formula (I) in which R₃ is —SO₃ ⁻ and p is 2, the —SO₃ ⁻ groups are preferably in the 2,5-positions of the phenyl ring.

In compounds of formula (I) in which R₃ is —CO₂ ⁻ and p is 1, the —CO₂ ⁻ group is preferably in the 2 or 4 position of the phenyl ring.

Preferred compounds of formula (I) are those in which

the anionic charge on the brightener is balanced by a cationic charge composed of one or more identical or different cations selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium which is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium which is, di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical or mixtures of said compounds,

-   -   R₁ and R₁′ may be the same or different, and each is hydrogen,         C₁-C₄ linear or branched alkyl, C₂-C₄ linear or branched         hydroxyalkyl, CH₂CO₂ ⁻, CH₂CH₂CONH₂ or CH₂CH₂CN,     -   R₂ and R₂′ may be the same or different, and each is C₁-C₄         linear or branched alkyl, C₂-C₄ linear or branched hydroxyalkyl,         CH₂CO₂ ⁻, CH(CO₂ ⁻)CH₂CO₂ ⁻ or CH₂CH₂SO₃ ⁻,     -   R₃ signify hydrogen, —CO₂ ⁻ or —SO₃ ⁻ and     -   p is 0, 1 or 2.

More preferred compounds of formula (I) are those in which

the anionic charge on the brightener is balanced by a cationic charge composed of one or more identical or different cations selected from the group consisting of Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, ammonium which is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium which is, di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical or mixtures of said compounds,

-   -   R₁ and R₁′ may be the same or different, and each is hydrogen,         methyl, ethyl, propyl, α-methylpropyl, β-methylpropyl,         β-hydroxyethyl, β-hydroxypropyl, CH₂CO₂ ⁻, CH₂CH₂CONH₂ or         CH₂CH₂CN,     -   R₂ and R₂′ may be the same or different, and each is methyl,         ethyl, propyl, α-methylpropyl, β-methylpropyl, β-hydroxyethyl,         β-hydroxypropyl, CH₂CO₂ ⁻, CH(CO₂ ⁻)CH₂CO₂ ⁻ or CH₂CH₂SO₃ ⁻,     -   R₃ signify hydrogen, —CO₂ ⁻ or —SO₃ ⁻ and     -   p is 0, 1 or 2.

Especially preferred compounds of formula (I) are those in which

the anionic charge on the brightener is balanced by a cationic charge composed of one or more identical or different cations selected from the group consisting of Na⁺, K⁺, triethanolammonium, N-hydroxyethyl-N,N-dimethylammonium, N-hydroxyethyl-N,N-diethylammonium or mixtures of said compounds,

-   -   R₁ and R₁′ may be the same or different, and each is hydrogen,         ethyl, propyl, 3-hydroxyethyl, p-hydroxypropyl, CH₂CO₂ ⁻, or         CH₂CH₂CONH₂,     -   R₂ and R₂′ may be the same or different, and each is ethyl,         propyl, β-hydroxyethyl, β-hydroxypropyl, CH₂CO₂ ⁻, CH(CO₂         ⁻)CH₂CO₂ ⁻, or CH₂CH₂SO₃ ⁻ and     -   R₃ signify hydrogen, —CO₂ ⁻ or —SO₃ ⁻ and     -   p is 0 or 1.

Compound of formula (I) is used in an amount typically of from 0.001 to 5% by weight, preferably in the range of from 0.01 to 3% by weight, the % by weight being based on the total weight of dry pulp.

Preferred compounds of formula (U) are those in which

-   -   R₄ signifies H, methyl or ethyl,     -   R₅ signifies paramethoxyphenyl, methyl or ethyl,     -   M signifies a cation selected from the group consisting of         hydrogen, an alkali metal cation, alkaline earth metal, ammonium         which is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear         or branched hydroxyalkyl radical, ammonium which is, di-, tri-         or tetrasubstituted by a mixture of C₁-C₄ linear or branched         alkylradical and linear or branched hydroxyalkyl radical or         mixtures of said compounds.

More preferred compounds of formula (II) are those in which

-   -   R₄ signifies methyl or ethyl,     -   R₅ signifies methyl or ethyl,     -   M signifies a cation selected from the group consisting of Li⁺,         Na⁺, K⁺, ½Ca²⁺, ½Mg²⁺, ammonium which is mono-, di-, tri- or         tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl         radical, ammonium which is, di-, tri- or tetrasubstituted by a         mixture of C₁-C₄ linear or branched alkylradical and linear or         branched hydroxyalkyl radical or mixtures of said compounds.

Especially preferred compounds of formula (U) are those in which

-   -   R₄ signifies methyl or ethyl,     -   R₅ signifies methyl or ethyl,     -   M signifies a cation selected from the group consisting of Na⁺,         K⁺, triethanolammonium, N-hydroxyethyl-N,N-dimethylammonium,         N-hydroxyethyl-N,N-diethylammonium or mixtures of said         compounds.

Compound of formula (II) is used in an amount typically of from 0.0001 to 0.02% by weight, preferably in the range of from 0.0005 to 0.01% by weight, the % by weight being based on the total weight of dry pulp.

The fibres in the pulp mass may be sourced from hardwood and/or softwood trees, and may comprise virgin fibres and/or recycled fibres. The fibres may be bleached or unbleached.

In addition to one or more compounds of formula (I), one or more compounds of formula (II) and water, the aqueous formulation may optionally contain one or more auxiliaries. Examples of such auxiliaries are for example antifreezers, dispersing agents, synthetic or natural thickeners, carriers, defoamers, wax emulsions, dyes, inorganic salts, solubilizing aids, preservatives, complexing agents, biocides, cross-linkers, pigments, special resins etc.

In addition to one or more compounds of formula (I), one or more compounds of formula (II), optionally one or more auxiliaries and water, the aqueous formulation may contain by-products formed during the preparation of compounds of formula (I) and compounds of formula (II).

The aqueous formulation may be prepared by mixing one or more compounds of formula (I), one or more compounds of formula (II) and optionally one or more auxiliaries as solids or as preformed aqueous solutions.

One or more compounds of formula (I), one or more compounds of formula (II), optionally one or more auxiliaries and water can be mixed in any order or at the same time to form the aqueous formulation.

Preferably, one or more compounds of formula (II), optionally one or more auxiliaries and water are added to a preformed aqueous solution containing one or more compounds of formula (I) to form the aqueous formulation.

When used as a preformed aqueous solution, the concentration of compound of formula (I) in water is preferably of from 1 to 50% by weight, more preferably of from 2 to 40% by weight, even more preferably from 10 to 30% by weight, the % by weight being based on the total weight of the preformed aqueous solution containing the compound of formula (I).

When used as a preformed aqueous solution, the concentration of compound of formula (II) in water is preferably of from 0.001 to 30% by weight, more preferably of from 0.01 to 25% by weight, even more preferably from 0.02 to 20% by weight, the % by weight being based on the total weight of the preformed aqueous solution containing the compound of formula (II).

The pH value of the aqueous formulation is typically in the range of from 5 to 13, preferably of from 6 to 11, more preferably of from 7 to 10. Where it is necessary to adjust the pH of the aqueous formulation, acids or bases may be employed. Examples of acids which may be employed include but are not restricted to hydrochloric acid, sulphuric acid, formic acid and acetic acid. Examples of bases which may be employed include but are not restricted to alkali metal and alkaline earth metal hydroxide or carbonates, ammonia or amines.

The present invention further provides a method for preparing white paper characterized in that the aqueous formulation containing one or more compounds of formula (I), one or more compounds of formula (II), optionally one or more auxiliaries and water is used.

The present invention therefore provides a method for preparing white paper characterized in that the aqueous formulation containing one or more compounds of formula (I), one or more compounds of formula (II), optionally one or more auxiliaries and water is added to the pulp mass prior to sheet formation.

The following examples shall demonstrate the instant invention in more details. In the present application, if not indicated otherwise, “parts” means “parts by weight” and “%” means “% by weight”.

EXAMPLES Preparative Example 1

An aqueous solution (S1) is prepared by slowly adding 79 parts of water to 921 parts of a preformed aqueous solution containing 0.216 mol per kg of compound of formula (1) (synthesized according to example 1 in GB 1114021 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.216 mol per kg of compound of formula (1)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous solution (S1) containing 0.199 mol per kg of compound of formula (1). The resulting aqueous solution (Si) has a pH in the range of from 8.0 to 9.0.

Preparative Example 1a

An aqueous formulation (F1a) is prepared by slowly adding 2 parts of compound of formula (a) and 77 parts of water to 921 parts of a preformed aqueous solution containing 0.216 mol per kg of compound of formula (1) (synthesized according to example 1 in GB 1114021 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.216 mol per kg of compound of formula (1)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F1a) containing compound of formula (a) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F1a) and 0.199 mol per kg of compound of formula (1). The resulting aqueous formulation (F1a) has a pH in the range of from 8.0 to 9.0.

Preparative Example 1b

An aqueous formulation (F1b) is prepared by slowly adding 2 parts of compound of formula (b) and 77 parts of water to 921 parts of a preformed aqueous solution containing 0.216 mol per kg of compound of formula (1) (synthesized according to example 1 in GB 1114021 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.216 mol per kg of compound of formula (1)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F1b) containing compound of formula (b) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F1b) and 0.199 mol per kg of compound of formula (1). The resulting aqueous formulation (F1b) has a pH in the range of from 8.0 to 9.0.

Comparative Example 1c

An aqueous formulation (F1c) is prepared by slowly adding 18.2 parts of a preformed aqueous solution containing 11 weight % of C.I. Direct Violet 35, the weight % being based on the total weight of the aqueous C.I. Direct Violet 35 preformed solution and 60.8 parts of water to 921 parts of a preformed aqueous solution containing 0.216 mol per kg of compound of formula (1) (synthesized according to example 1 in GB 1114021 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.216 mol per kg of compound of formula (1)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F1 c) containing C.I. Direct Violet 35 at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F1c) and 0.199 mol per kg of compound of formula (1). The resulting aqueous formulation (F1c) has a pH in the range of from 8.0 to 9.0.

Preparative Example 2

An aqueous solution (S2) is prepared by slowly adding 26 parts of water and 150 parts of urea to 824 parts of a preformed aqueous mixture containing 0.250 mol per kg of compound of formula (2), synthesized according to example 1 in EP 0884312-A1, at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous solution (S2) containing 0.206 mol per kg of compound of formula (2). The resulting aqueous solution (S2) has a pH in the range of from 8.0 to 9.0.

Preparative Example 2a

An aqueous formulation (F2a) is prepared by slowly adding 2 parts of compound of formula (a), 150 parts of urea and 24 parts of water to 824 parts of a preformed aqueous mixture containing 0.250 mol per kg of compound of formula (2) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F2a) containing compound of formula (a) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F2a) and 0.206 mol per kg of compound of formula (2). The resulting aqueous formulation (F2a) has a pH in the range of from 8.0 to 9.0.

Preparative Example 2b

An aqueous formulation (F2b) is prepared by slowly adding 2 parts of compound of formula (b), 150 parts of urea and 24 parts of water to 824 parts of a preformed aqueous mixture containing 0.250 mol per kg of compound of formula (2) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F2b) containing compound of formula (b) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F2b) and 0.206 mol per kg of compound of formula (2). The resulting aqueous formulation (F2b) has a pH in the range of from 8.0 to 9.0.

Comparative Example 2c

An aqueous formulation (F2c) is prepared by slowly adding 18.2 parts of a preformed aqueous solution containing 11 weight % of C.l. Direct Violet 35, the weight % being based on the total weight of the aqueous C.I. Direct Violet 35 preformed solution, 150 parts of urea and 7.8 parts of water to 824 parts of a preformed aqueous mixture containing 0.250 mol per kg of compound of formula (2) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F2c) containing C.I. Direct Violet 35 at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F2c) and 0.206 mol per kg of compound of formula (2). The resulting aqueous formulation (F2c) has a pH in the range of from 8.0 to 9.0.

Preparative Example 3

An aqueous solution (S3) is prepared by slowly adding 181 parts of water to 819 parts of a preformed aqueous mixture containing 0.238 mol per kg of compound of formula (3), synthesized according to example 1 in WO 2007/017336-A1, at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous solution (S3) containing 0.195 mol per kg of compound of formula (3). The resulting aqueous solution (S3) has a pH in the range of from 8.0 to 9.0.

Preparative Example 3a

An aqueous formulation (F3a) is prepared by slowly adding 2 parts of compound of formula (a) and 179 parts of water to 819 parts of a preformed aqueous mixture containing 0.238 mol per kg of compound of formula (3) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F3a) containing compound of formula (a) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F3a) and 0.195 mol per kg of compound of formula (3). The resulting aqueous formulation (F3a) has a pH in the range of from 8.0 to 9.0.

Preparative Example 3b

An aqueous formulation (F3b) is prepared by slowly adding 2 parts of compound of formula (b) and 179 parts of water to 819 parts of a preformed aqueous mixture containing 0.238 mol per kg of compound of formula (3) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F3b) containing compound of formula (b) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F3b) and 0.195 mol per kg of compound of formula (3). The resulting aqueous formulation (F3b) has a pH in the range of from 8.0 to 9.0.

Comparative Example 3c

An aqueous formulation (F3c) is prepared by slowly adding 18.2 parts of a preformed aqueous solution containing 11 weight % of C.I. Direct Violet 35, the weight % being based on the total weight of the aqueous C.I. Direct Violet 35 preformed solution and 162.8 parts of water to 819 parts of a preformed aqueous mixture containing 0.238 mol per kg of compound of formula (3) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F3c) containing C.I. Direct Violet 35 at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F3c) and 0.195 mol per kg of compound of formula (3). The resulting aqueous formulation (F3c) has a pH in the range of from 8.0 to 9.0.

Preparative Example 4

An aqueous solution (84) is prepared by slowly adding 157 parts of water to 843 parts of a preformed aqueous mixture containing 0.210 mol per kg of compound of formula (4) (synthesized according to example 1 in WO 2011/033064-A2 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.210 mol per kg of compound of formula (4)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous solution (S4) containing 0.177 mol per kg of compound of formula (4). The resulting aqueous solution (S4) has a pH in the range of from 8.0 to 9.0.

Preparative Example 4a

An aqueous formulation (F4a) is prepared by slowly adding 2 parts of compound of formula (a) and 155 parts of water to 843 parts of a preformed aqueous mixture containing 0.210 mol per kg of compound of formula (4) (synthesized according to example 1 in WO 2011/033064-A2 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.210 mol per kg of compound of formula (4)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F4a) containing compound of formula (a) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F4a) and 0.177 mol per kg of compound of formula (4). The resulting aqueous formulation (F4a) has a pH in the range of from 8.0 to 9.0.

Preparative Example 4b

An aqueous formulation (F4b) is prepared by slowly adding 2 parts of compound of formula (b) and 155 parts of water to 843 parts of a preformed aqueous mixture containing 0.210 mol per kg of compound of formula (4) (synthesized according to example 1 in WO 2011/033064-A2 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.210 mol per kg of compound of formula (4)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F4b) containing compound of formula (b) at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F4b) and 0.177 mol per kg of compound of formula (4). The resulting aqueous formulation (F4b) has a pH in the range of from 8.0 to 9.0.

Comparative Example 4c

An aqueous formulation (F4c) is prepared by slowly adding 18.2 parts of a preformed aqueous solution containing 11 weight % of C.I. Direct Violet 35, the weight % being based on the total weight of the aqueous C.I. Direct Violet 35 preformed solution and 138.8 parts of water to 843 parts of a preformed aqueous mixture containing 0.210 mol per kg of compound of formula (4) (synthesized according to example 1 in WO 2011/033064-A2 with the sole difference that the final solution was ultra-filtered to remove salts and concentrated to 0.210 mol per kg of compound of formula (4)) at room temperature with efficient stirring. The obtained mixture is stirred for 1 hour at room temperature to afford 1000 parts of an aqueous formulation (F4c) containing C.I. Direct Violet 35 at a concentration of 0.2 weight %, the weight % being based on the total weight of the final aqueous formulation (F4c) and 0.177 mol per kg of compound of formula (4). The resulting aqueous formulation (F4c) has a pH in the range of from 8.0 to 9.0.

Application Example 1

The solution (S1) and the formulations (F1a), (F1b) and (F1c) prepared according to preparative Example 1, 1a, 1b and comparative example 1c respectively are added at a range of concentrations from 0 to 2 weight %, the weight % being based on the total weight of dry fibre to 200 parts of a 2.5% aqueous suspension of a 50:50 mixture of bleached spruce sulphite cellulose and bleached beech sulphite cellulose beaten to a Schopper Riegler wetness of 20° SR. The suspensions are stirred for 5 minutes, then diluted to 1000 parts. A paper sheet is then made by drawing the suspension through a wire mesh. After being pressed and dried, the paper is measured for whiteness and brightness on a calibrated Minolta spectrophotometer.

Results are depicted in table 1a and 1b respectively and clearly shows the significant improvement in whiteness while avoiding the disadvantages characterized by the use of shading dyes (loss of brightness).

TABLE 1a CIE Whiteness Solution (S1) Formulation (F1a) Formulation (F1b) from preparative from preparative from preparative Conc. % example 1 example 1a example 1b 0.0 64.1 64.1 64.1 0.4 109.7 114.9 113.5 0.8 119.6 124.2 123.9 1.2 124.2 131.3 129.5 1.6 126.8 134.1 133.1 2.0 125.7 136.6 135.5

TABLE 1b Brightness Formulation Formulation Solution (S1) Formulation (F1a) (F1b) from (F1c) from Conc. from preparative from preparative preparative comparative % example 1 example 1a example 1b example 1c 0.0 84.4 84.4 84.4 84.4 0.4 100.3 101.3 101.0 98.5 0.8 104.6 104.4 104.7 99.7 1.2 107.0 106.8 106.9 100.3 1.6 108.5 107.8 107.8 99.5 2.0 108.6 108.3 108.9 98.8

Application Example 2

The solution (S2) and the formulations (F2a), (F2b) and (F2c) prepared according to preparative Example 2, 2a, 2b and comparative example 2c respectively are added at a range of concentrations from 0 to 2 weight %, the weight % being based on the total weight of dry fibre to 200 parts of a 2.5% aqueous suspension of a 50:50 mixture of bleached spruce sulphite cellulose and bleached beech sulphite cellulose beaten to a Schopper Riegler wetness of 20° SR. The suspensions are stirred for 5 minutes, then diluted to 1000 parts. A paper sheet is then made by drawing the suspension through a wire mesh. After being pressed and dried, the paper is measured for whiteness on a calibrated Minolta spectrophotometer.

Results are depicted in table 2a and 2b respectively and clearly shows the significant improvement in whiteness while avoiding the disadvantages characterized by the use of shading dyes (loss of brightness).

TABLE 2a CIE Whiteness Solution (S2) Formulation (F2a) Formulation (F2b) from preparative from preparative from preparative Conc. % example 2 example 2a example 2b 0.0 64.1 64.1 64.1 0.4 112.5 116.8 115.4 0.8 120.3 124.4 123.9 1.2 125.2 128.7 128.7 1.6 126.0 131.3 131.7 2.0 127.7 133.4 132.9

TABLE 2b Brightness Formulation Formulation Solution (S2) Formulation (F2a) (F2b) from (F2c) from Conc. from preparative from preparative preparative comparative % example 2 example 2a example 2b example 2c 0.0 84.4 84.4 84.4 84.4 0.4 101.5 102.6 102.0 100.1 0.8 105.0 105.3 105.0 101.2 1.2 107.5 106.8 106.7 101.0 1.6 108.2 107.7 107.9 100.7 2.0 109.3 108.2 108.1 99.6

Application Example 3

The solution (S3) and the formulations (F3a), (F3b) and (F3c) prepared according to preparative Example 3, 3a, 3b and comparative example 3c respectively are added at a range of concentrations from 0 to 2 weight %, the weight % being based on the total weight of dry fibre to 200 parts of a 2.5% aqueous suspension of a 50:50 mixture of bleached spruce sulphite cellulose and bleached beech sulphite cellulose beaten to a Schopper Riegler wetness of 20° SR. The suspensions are stirred for 5 minutes, then diluted to 1000 parts. A paper sheet is then made by drawing the suspension through a wire mesh. After being pressed and dried, the paper is measured for whiteness on a calibrated Minolta spectrophotometer.

Results are depicted in table 3a and 3b respectively and clearly shows the significant improvement in whiteness while avoiding the disadvantages characterized by the use of shading dyes (loss of brightness).

TABLE 3a CIE Whiteness Solution (S3) Formulation (F3a) Formulation (F3b) from preparative from preparative from preparative Conc. % example 3 example 3a example 3b 0.0 64.1 64.1 64.1 0.4 112.3 116.4 116.1 0.8 119.7 126.6 125.6 1.2 123.9 132.0 130.7 1.6 126.1 135.1 134.2 2.0 126.1 138.4 135.3

TABLE 3b Brightness Formulation Formulation Solution (S3) Formulation (F3a) (F3b) from (F3c) from Conc. from preparative from preparative preparative comparative % example 3 example 3a example 3b example 3c 0.0 84.4 84.4 84.4 84.4 0.4 101.4 101.9 102.0 99.4 0.8 104.7 105.6 105.1 100.0 1.2 106.9 107.3 106.9 100.5 1.6 108.4 108.1 107.9 99.3 2.0 108.7 108.8 107.8 98.3

Application Example 4

The solution (S4) and the formulations (F4a), (F4b) and (F4c) prepared according to preparative Example 4, 4a, 4b and comparative example 4c respectively are added at a range of concentrations from 0 to 2 weight %, the weight % being based on the total weight of dry fibre to 200 parts of a 2.5% aqueous suspension of a 50:50 mixture of bleached spruce sulphite cellulose and bleached beech sulphite cellulose beaten to a Schopper Riegler wetness of 20° SR. The suspensions are stirred for 5 minutes, then diluted to 1000 parts. A paper sheet is then made by drawing the suspension through a wire mesh. After being pressed and dried, the paper is measured for whiteness on a calibrated Minolta spectrophotometer.

Results are depicted in table 4a and 4b respectively and clearly shows the significant improvement in whiteness while avoiding the disadvantages characterized by the use of shading dyes (loss of brightness).

TABLE 4a CIE Whiteness Solution (S4) Formulation (F4a) Formulation (F4b) from preparative from preparative from preparative Conc. % example 4 example 4a example 4b 0.0 64.1 64.1 64.1 0.4 109.1 116.5 114.4 0.8 119.7 126.0 126.9 1.2 125.0 131.6 132.3 1.6 125.7 132.3 133.7 2.0 126.1 133.9 135.2

TABLE 4b Brightness Formulation Formulation Solution (S4) Formulation (F4a) (F4b) from (F4c) from Conc. from preparative from preparative preparative comparative % example 4 example 4a example 4b example 4c 0.0 85.5 85.5 85.5 85.5 0.4 99.9 101.9 100.8 99.5 0.8 104.5 105.1 105.5 100.9 1.2 107.8 107.1 107.4 100.5 1.6 108.6 107.3 107.8 99.5 2.0 109.1 107.5 108.2 99.1 

1. A method for preparing white paper which comprising the step of adding to the pulp mass an aqueous formulation comprising: a) at least one optical brightener of formula (I)

the anionic charge on the at least one optical brightener is balanced by a cationic charge including of one or more identical or different cations selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium that is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched alkyl radical, ammonium that is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium that is di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical and mixtures thereof, and b) at least one shading dye of formula (II)

c) optionally one or more auxiliaries and d) water.
 2. A method according to claim 1, wherein in the at least one optical brightener of formula (I) R₃ is —SO₃ ⁻ and p is 1 and the —SO₃ ⁻ group is in the 4-position of the phenyl ring.
 3. The method according to claim 1, wherein in the at least one optical brightener formula (I), R₃ is —SO₃ ⁻, p is 2 and the —SO₃ ⁻ groups are in the 2,5-positions of the phenyl ring.
 4. The method according to claim 1, wherein in the at least one optical brightener of formula (I), R₃ is —CO_(2hu −), p is 1 and the —CO₂ ⁻ group is in the 2 or 4 position of the phenyl ring.
 5. The method according to claim 1, wherein in the at least one optical brightener of formula (I) the anionic charge on the at least one optical brightener is balanced by a cationic charge including one or more identical or different cations selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium that is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium which that is, di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical and mixtures thereof, and R₁ and R₁′ are the same or different, and each is hydrogen, C₁-C₄ linear or branched alkyl, C₂-C₄ linear or branched hydroxyalkyl, CH₂CO₂ ⁻, CH₂CH₂CONH₂ or CH₂CH₂CN, and R₂ and R₂′ are the same or different, and each is C₁-C₄ linear or branched alkyl, C₂-C₄ linear or branched hydroxyalkyl, CH₂CO₂ ⁻, CH(CO₂)CH₂CO₂ or CH₂CH₂SO₃ ⁻, R₃ is hydrogen, —CO₂ ⁻ or —SO₃ ⁻ and p is 0, 1 or
 2. 6. The method according to claim 1, wherein in the at least one optical brightener of formula (I) the anionic charge on the at least one optical brightener is balanced by a cationic charge including one or more identical or different cations selected from the group consisting of Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, ammonium that is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium that is, di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical and mixtures thereof, R₁ and R₁′ are the same or different, and each is hydrogen, methyl, ethyl, propyl, α-methylpropyl, β-methylpropyl, β-hydroxyethyl, β-hydroxypropyl, CH₂CO₂ ⁻, CH₂CH₂CONH₂ or CH₂CH₂CN, R₂ and R₂′ are the same or different, and each is methyl, ethyl, propyl, α-methylpropyl, β-methylpropyl, β-hydroxyethyl, β-hydroxypropyl, CH₂CO₂ ⁻, CH(CO₂ ⁻)CH₂CO₂ ⁻ or CH₂CH₂SO₃ ⁻, R₃ is hydrogen, —CO₂ ⁻ or —SO₃ ⁻ and p is 0, 1 or
 2. 7. The method according to claim 1, wherein in the at least one optical brightener of formula (I) the anionic charge on the brightener is balanced by a cationic charge including one or more identical or different cations selected from the group consisting of Na⁺, K⁺, triethanolammonium, N-hydroxyethyl-N,N-dimethylammonium, N-hydroxyethyl-N,N-diethylammonium and mixtures thereof, R₁ and R₁′ are the same or different, and each is hydrogen, ethyl, propyl, β-hydroxyethyl, β-hydroxypropyl, CH₂CO₂ ⁻, or CH₂CH₂CONH₂, R₂ and R₂′ are the same or different, and each is ethyl, propyl, β-hydroxyethyl, β-hydroxypropyl, CH₂CO₂ ⁻, CH(CO₂)CH₂CO₂ ⁻ or CH₂CH₂SO₃ ⁻ and R₃ is hydrogen, —CO₂ ⁻ or —SO₃ ⁻ and p is 0 or
 1. 8. The method according to claim 1, wherein the at least one optical brightener of formula (I) is used in an amount from 0.001 to 5% by weight, the % by weight being based on the total weight of dry pulp.
 9. The method according to claim 1, wherein in the at least one optical brightener of formula (II) R₄ is H, methyl or ethyl, R₅ is paramethoxyphenyl, methyl or ethyl, M is a cation selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium that is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium that is, di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical and mixtures thereof.
 10. The method according to claim 1, wherein in the at least one shading dye of formula (II) R₄ is methyl or ethyl, R₅ is methyl or ethyl, M is a cation selected from the group consisting of Li⁺, Na⁺, K⁺, ½ Ca²⁺, ½ Mg²⁺, ammonium that is mono-, di-, tri- or tetrasubstituted by a C₁-C₄ linear or branched hydroxyalkyl radical, ammonium that is di-, tri- or tetrasubstituted by a mixture of C₁-C₄ linear or branched alkylradical and linear or branched hydroxyalkyl radical and mixtures thereof.
 11. The method according to claim 1, wherein in the at least one shading dye of formula (II) R₄ is methyl or ethyl, R₅ is methyl or ethyl, M is a cation selected from the group consisting of Na⁺, K⁺, triethanolammonium, N-hydroxyethyl-N,N-dimethylammonium, N-hydroxyethyl-N,N-diethylammonium and mixtures thereof.
 12. The method according to claim 1, wherein the at least one shading dye of formula (II) is used in an amount from 0.0001 to 0.02% by weight, the % by weight being based on the total weight of dry pulp.
 13. The method according to claim 1, wherein the aqueous formulation contains one or more auxiliaries selected from the group consisting of antifreezers, dispersing agents, synthetic or natural thickeners, carriers, defoamers, wax emulsions, dyes, inorganic salts, solubilizing aids, preservatives, complexing agents, biocides, cross-linkers, pigments and special resins.
 14. The method according to claim 1, wherein the adding step further comprises mixing the at least one optical brightener of formula (I), the at least one shading dye of formula (II) and optionally one or more auxiliaries, wherein the at least one optical brightener, the at least one shading dye and the optional one or more auxiliaries are solids or preformed aqueous solutions.
 15. The method according to claim 14, wherein in the aqueous solution, the concentration of compound of formula (I) in water is of from 1 to 50% by weight, the % by weight being based on the total weight of the preformed aqueous solution containing the at least one optical brightener of formula (I).
 16. The method according to claim 14, wherein in the aqueous solution, the concentration of the at least one shading dye of formula (II) in water is from 0.001 to 30% by weight, the % by weight being based on the total weight of the preformed aqueous solution containing the the at least one shading dye of formula (II).
 17. The method according to claim 14, wherein the pH value of the aqueous formulation is in the range of from 5 to
 13. 18. The method according to claim 1, wherein the at least one optical brightener of formula (I) is used in an amount from 0.01 to 3% by weight, the % by weight being based on the total weight of dry pulp.
 19. The method according to claim 1, wherein the at least one shading dye of formula (II) is used in an amount from 0.0005 to 0.01% by weight, the % by weight being based on the total weight of dry pulp. 